Asymmetric cross-hemispheric climate response to extratropical forcing mediated by tropical Pacific coupling

Regional climate perturbations can influence distant regions through coupled ocean-atmosphere processes. However, how forcing in one hemisphere affects the other remains underexplored. Here, using Earth system model simulations, we show that Southern Hemisphere extratropical cooling induces a robust cross-hemispheric temperature response, particularly over the Arctic, whereas equivalent Northern Hemisphere cooling yields a localized response. This asymmetry initiated from differences in how extratropical cooling interacts with the tropical Pacific. Southern Hemisphere forcing more effectively cools the equatorial central Pacific through enhanced surface wind-evaporative feedback, leading to stronger suppression of deep convection. It generates a planetary-scale Rossby wave train that establishes low-pressure anomalies over the Arctic, which reduce subsidence and adiabatic warming, thus promoting sea ice growth and amplifying surface cooling. In contrast, weaker tropical response under Northern Hemisphere forcing limits cross-hemispheric influence. Our findings highlight the central role of tropical Pacific processes in shaping asymmetry in hemispheric climate interaction. Southern Hemisphere extratropical cooling induces strong temperature responses in the Northern Hemisphere whereas equivalent Northern Hemisphere cooling remains more localized, primarily due to hemispheric asymmetries in tropical air-sea coupling, revealed by climate model experiments.